In some heretoforeknown filling machines the dosage (metering) of the products, during filling of the containers, is carried out by a dosage device comprising a double-acting piston sliding in a cylindrical chamber into which the product is made to pass before it can reach the delivery apparatus. The dosage of the product is directly dependent on the displacement of the double acting piston and on the consequent variation of volume in the two parts of the cylindrical chamber which are at one at either side of the piston; in the phase where the product is drawn into one part of the chamber there is an outflow of the product from the other part of the chamber. Machines of the type described provided with dosage delivery means are already well known on the market and their names are known to those skilled in the art of volumetric filling machines.
One problem, among others, which the machine manufacturers have had to face has been that of reducing, so far as possible, errors in the dosage of the product.
The accuracy of the dosage, in known machines, is extremely sensitive to loss of sealing of the various parts of the apparatus for the control and distribution of the product (with consequent loss of product) and also infiltration of foreign fluids into the conduits for the feeding and distribution of the product from the feeding source to the dosage delivery apparatus. Possible infiltrations, of air for example, may in fact be the cause of variations by reduction of the flow of the product into the dosage apparatus, with consequent errors in dosage. In order not to have, on discharge from the known filling machines, containers filled with a quantity of the product even slightly less than that required (for many products the total contents must be declared on the label which marks them), users of such machines usually resort to a setting in excess of the volumetric quantity of the product required. For low cost products this setting leads to negligible (or at least tolerable) losses in relation to production costs, but in the case of more expensive products over long operative periods, the cost can be considerable.
In attempting to solve the problem of dosage control particular attention has been paid to sealing, for example in the valves for control and distribution of the product to the dosage/delivery apparatus. Thus particular valves suitable for use in a dosage/delivery apparatus, have been proposed, commonly known by the term "spool valve". Such valves include a generally cylindrical body sliding inside a cylindrical chamber into which open both supply conduit for the fluid to be dosed (metered) and the conduit for feeding the product to the dosage apparatus and then to the delivery apparatus. Such a cylindrical body, or slide, is shaped in such a way as to present regions of reduced diameter which define corresponding chambers (generally three) which in conjunction with the position of the slide inside the cylindrical chamber, allow the intercommunication with each other of the above-mentioned conduits, to obtain the operative phases of dosage and delivery. In some cases the parts of the slide which delimit said chambers are equipped with one or more peripheral sealing rings designed to seal against the internal walls of the cylindrical chamber as the slide slides to and fro in the chamber; in others the cylindrical chamber has annular seatings in which sealing rings are housed to seal against those surfaces of the parts of the slide which are not of a reduced diameter. Such valves are normally operated pneumatically by admitting air under pressure directly into the cylindrical chamber so as to cause axial strokes of the slide to and fro inside the cylindrical chamber. In the presence of fluid products with chemically corrosive characteristics, or fluid products with aggressively abrasive characteristics, sealing elements are subject to rapid wear, giving rise to the disadvantages described above. These disadvantages are more likely to arise in high-speed filling machines equipped with multiple dosage/delivery devices and where there is a considerable operating pressure both in the control and distribution circuits for the product and in the pneumatic control circuits for the valves associated with the various dosage/delivery devices.